1. Field of the Invention
The present invention relates to an apparatus for measuring Optical Beat Interference (OBI) noise that is applied to a Central Office (CO) included in a Subcarrier Multiple Access (SCMA) optical network, and more particularly to an apparatus for measuring OBI noise in the SCMA optical network, wherein OBI noise in a signal output from an optical receiver of the central office in the optical network is measured in both a low band of frequencies below a band of subcarrier signals and a high band of frequencies above the band of subcarrier signals, thereby making it possible to quickly and correctly detect the occurrence of OBI noise and quickly and correctly measure the level of the OBI noise, regardless of which band the OBI noise occurs in.
2. Description of the Related Art
Optical networks have received a great deal of attention as next-generation subscriber access networks for the information age. A point-to-point optical network can provide a large amount of data to subscribers with high security. Despite this advantage, the point-to-point optical network has not yet been commercialized due to severe implementation costs.
One economical optical network is a point-to-multipoint optical network that allows a number of subscribers to share a single optical fiber and decreases network implementation costs per subscriber.
The point-to-multipoint optical network can be implemented using a Subcarrier Multiple Access (SCMA) scheme in which different subcarriers are allocated to optical signals of subscribers sharing a single optical fiber. In the SCMA scheme, a subscriber loads its information on a subcarrier allocated to the subscriber to transmit it, and the central office uses a band pass filter corresponding to the subscriber to pass a signal received from the subscriber to extract the information of the subscriber.
FIG. 1 is a schematic block diagram of a conventional point-to-point optical network.
As shown in FIG. 1, the conventional point-to-point optical network comprises a plurality of subscriber terminals 10-1 to 10-N, a plurality of optical fibers (OF), and a Central Office (CO) 20. The plurality of subscriber terminals 10-1 to 10-N include a plurality of optical transmitters 11-1 to 11-N for transmitting a plurality of optical signals, respectively. The plurality of optical fibers (OF) carry the plurality of optical signals transmitted from the plurality of optical transmitters 11-1 to 11-N, respectively. The Central Office (CO) 20 includes an optical receiver 21 for receiving the plurality of optical signals from the plurality of optical fibers (OF) through different inputs. Here, the subscriber terminals 10-1 to 10-N correspond to Subscriber Optical Network Terminals (ONTs) or Optical Network Units (ONUs), and the optical receiver 21 in the central office 20 corresponds to a telephone office Optical Line Terminal (OLT).
However, since it requires a number of optical fibers, the point-to-point optical network has high implementation costs, increasing costs per subscriber. A point-to-multipoint optical network as shown in FIG. 2 has been suggested to overcome this problem.
FIG. 2 is a schematic block diagram of a conventional point-to-multipoint optical network.
The conventional point-to-multipoint network shown in FIG. 2 is a network that is implemented by applying the SCMA scheme to the point-to-point optical network in order to allow a number of subscribers to share a single optical fiber.
As shown in FIG. 2, the conventional point-to-multipoint optical network comprises a plurality of subscriber terminals 30-1 to 30-N, a plurality of first optical fibers (OF1), an optical coupler 40, a second optical fiber (OF2), and a central office 50. The plurality of subscriber terminals 30-1 to 30-N include optical transmitters 31-1 to 31-N for transmitting a plurality of optical signals, respectively. The plurality of first optical fibers (OF1) carry the plurality of optical signals, transmitted from the plurality of optical transmitters 31-1 to 31-N, to the optical coupler 40. The optical coupler 40 combines the plurality of optical signals passed through respective ones of the plurality of first optical fibers (OF1) into a single optical signal. The second optical fiber (OF2) carries the optical signal output from the optical coupler 40 to the central office 50. The central office-50 includes an optical receiver 51 for receiving the optical signal passed through the second optical fiber (OF2) through a single input.
The plurality of subscriber terminals 30-1 to 30-N load their information on different subcarriers for transmission. The optical receiver 51 of the central office 50 discriminates and processes signals received from the subscriber terminals according to their subcarriers. Since the plurality of subscribers share the second optical fiber (OF2) based on the SCMA scheme, it is possible to decrease costs per subscriber, thereby achieving a low-cost optical network.
However, in the SCMA optical network, optical beat interference (OBI) occurs if the optical receiver 51 in the central office 50 simultaneously receives two or more optical signals. If OBI noise is present in the band of subcarrier signals, the OBI noise is a major factor decreasing Signal to Noise Ratio (SNR).
The central frequency of OBI noise corresponds to the difference between the central frequencies of two received optical signals, and the spectrum of the OBI noise has a form similar to that of the convolution of the spectrums of the two optical signals. Such OBI noise occurs in an optical receiver if the optical receiver simultaneously receives a large number of optical signals as in the SCMA optical network.
In other words, if a frequency corresponding to the difference between the central frequencies of two received optical signals is present in the band of subcarrier signals, OBI noise occurs in the band of subcarrier signals, reducing the signal to noise ratio. To guarantee QoS in the SCMA optical network, it is necessary to quickly detect the occurrence of OBI when the OBI occurs, so as to control a light source causing the OBI.
A conventional method for measuring OBI noise is described below.
FIG. 3 is a block diagram of an optical network in which a conventional Optical Beat Interference (OBI) measurement apparatus is provided.
As shown in FIG. 3, the optical network comprises a receiving station 1, a plurality of transmitting stations, and an optical coupler 4. The plurality of transmitting stations 2 transmit a plurality of optical signals to the receiving station 1 through the optical coupler 4. The receiving station 1 comprises an optical receiver 15, a plurality of filters (f1 to f3) 8, a plurality of demodulators 9, and a conventional OBI noise measurement apparatus including an OBI noise filter 10 and a noise meter 11. The optical receiver 15 receives an optical signal from the optical coupler 4, and the plurality of filters 8 pass only corresponding subcarriers of the output signal of the optical receiver 15, respectively. The plurality of demodulators 9 demodulate signals output from the filters 8, respectively. The OBI noise filter 10 passes a specified band of frequencies of the output signal of the optical receiver 15 other than the subcarrier band. The noise meter 11 measures noise passed through the OBI noise filter 10.
In such an optical network including the conventional OBI noise measurement apparatus as shown in FIG. 3, the receiving station (corresponding to the central office) passes noise in a specified band of frequencies other than the subcarrier band through the noise filter 10, and uses the noise meter to continuously measure the power of noise passed through the noise filter 11. The conventional OBI noise measurement apparatus performs OBI noise measurement, based on a property of the noise filter that the output noise power of the filter is increased if Optical Beat Interference (OBI) noise occurs due to beating between two or more optical signals in an optical network, thereby affecting the band of subcarriers.
The conventional OBI measurement apparatus is described in detail in United Kingdom Patent Publication No. GB 2 294 372 A.
In another conventional OBI measurement method, the power of noise in a specified band of frequencies other than the band of subcarriers is measured in the same manner as in the OBI measurement method of FIG. 3, and the power of each subcarrier signal is also measured to determine OBI noise power relative to signal power, rather than absolute noise power, so as to operate light sources to operate under a condition maximizing the signal-to-noise power ratio.
However, the conventional OBI measurement apparatus and methods have the following problems.
The power of OBI noise is measured in a low frequency band below the subcarrier band or in a high frequency band above the subcarrier band, measurements of the power of OBI noise vary depending on whether OBI noise occurs in the low frequency band below the subcarrier band or in the high frequency band above the subcarrier band.
For example, in the case where two received optical signals have been modulated with a modulation index of 0.1 using two subcarrier signals of 2 GHz and 3 GHz, respectively, the difference between central frequencies of the two optical signals is 4 GHz, and the noise filter provided for measuring OBI noise passes a band of frequencies of 2 GHz or less, the power of OBI noise occurring in the band of frequencies of 2 GHz or less is lower than the power of OBI noise measured in a band of frequencies of 3 GHz or more.
In addition, in the case where two received optical signals have been modulated using two subcarrier signals of 2 GHz and 3 GHz, respectively, the difference between central frequencies of the two optical signals is 1 GHz, and the filter provided for measuring OBI noise passes a band of frequencies of 3 GHz or more, the power of OBI noise measured in a band of frequencies of 3 GHz or more is lower than the power of OBI noise occurring in a band of frequencies of 2 GHz or less.
This not only causes an OBI noise measurement error but also makes it difficult to quickly and correctly detect the occurrence of OBI noise and quickly and correctly measure the level of the OBI noise in the case where the OBI noise occurs in the subcarrier band or at a frequency near the subcarrier band.